- Slides: 11
Chapter 18 Volcanic Activity
Magma • A mixture of molten rock, suspended mineral grains, and dissolved gases deep beneath Earth’s surface. • Forms when temperatures are high enough to melt rock – 800°C-1200°C
Factors That Affect Magma Formation • Temperature and Pressure – As pressure increases the temperature at which a substance melts increases. • Presence of water – Lowers the melting point temperature
Three Major Types of Magma 1. Basaltic magma – Least explosive 2. Andesitic magma 3. Rhyolitic magma – Most explosive • Magmas differ in the source rock from which they form, viscosity, silica content, gas content and explosiveness
Intrusive Activity Affects Earth’s Crust • Magma can force rock apart and enter newly formed fissures (cracks) • Magma can cause rocks to break off and sink into the magma chamber • Magma can melt rock into which it intrudes
Types of Intrusions • Batholiths, stocks, sill, dikes, and laccoliths are plutons that are classified according to their size shape and relationship to surrounding rock. • Batholiths are the largest plutons and often form the cores of many of Earth’s major mountain chains.
Volcanoes • Lava flows onto Earth’s surface through a vent • Multiple lava flows can accumulate to form a volcano • A crater is a depression that forms around a vent at the summit of a volcano • A caldera is a large crater that forms when a volcano collapses during or after an eruption.
Three Types of Volcanoes • Shield volcanoes • Cinder cone volcanoes • Composite cone volcanoes
Tephra • Rock Fragments ejected from a volcano
Where Volcanoes Form • Convergent and divergent • Flood basalts plate boundaries – when lava flows from fissures to form flat • Hotspots – Usually hot areas in the mantle that are stationary for a long period of time plains or plateaus